Piezoelectric transducer

ABSTRACT

A miniature piezoelectric transducer element is provided, comprising: (a) a cell element having a cavity; (b) a flexible piezoelectric layer attached to the cell member, the piezoelectric layer having an external surface and an internal surface, the piezoelectric layer featuring such dimensions so as to enable fluctuations thereof at its resonance frequency upon impinging of an external acoustic wave; and (c) a first electrode attached to the external surface and a second electrode attached to the internal surface of the piezoelectric layer. At least one of the electrodes may be specifically shaped so as to provide a maximal electrical output, wherein the electrical output may be current, voltage or power. A preferred shape of the electrodes includes two cores interconnected by a connecting member. The transducer element may function as a transmitter. When used as a transmitter, the electrodes are electrically connected to an electrical circuit including a switching element for modulating the reflected acoustic wave by controllably changing the mechanical impedance of the piezoelectric layer according to the frequency of an electrical message signal arriving from an electronic member, such as a sensor. Third and fourth electrodes may be attached to the piezoelectric layer and the electrical circuit, such that the switching element alternately connects the electrodes in parallel and anti-parallel electrical connections so as to controllably change the mechanical impedance of the piezoelectric layer.

[0001] This is a continuation of U.S. patent application Ser. No.09/691,887, filed Oct. 20, 2000, which is a continuation of U.S. patentapplication Ser. No. 09/000,553, filed Dec. 30, 1997, now U.S. Pat. No.6,140,740, issued Oct. 31, 2000.

FIELD AND BACKGROUND OF THE INVENTIO

[0002] The present invention relates to an acoustic transducer and, inparticular, to a miniature flexural piezoelectric transducer forreceiving acoustic energy transmitted from a remote source andconverting such energy into electrical power for activating anelectronic circuit. Further, the present invention relates to aminiature flexural piezoelectric transmitter for transmitting acousticinformation by modulating the reflection of an external impingingacoustic wave.

[0003] The prior art provides various examples of piezoelectrictransducers. Examples of such piezoelectric transducers are disclosed inU.S. Pat. Nos. 3,792,204; 4,793, 825; 3,894,198; 3,798,473, and4,600,855.

[0004] However, none of the prior art references provides a miniatureflexural piezoelectric transducer specifically tailored so as to allowthe usage of low frequency acoustic signals for vibrating thepiezoelectric layer at its resonant frequency, wherein substantially lowfrequency signals herein refer to signals having a wavelength that ismuch larger than the dimensions of the transducer. Further, none of theprior art references provides a miniature transducer having electrodesspecifically shaped so as to maximize the electrical output of thetransducer. Further, none of the above references provides a transducerelement which may be integrally manufactured with any combination ofelectronic circuits by using photolithographic and microelectronicstechnologies.

[0005] Further, the prior art fails to provide a miniature flexuralpiezoelectric transmitter which modulates the reflected acoustic wave bycontrollably changing the mechanical impedance of the piezoelectriclayer according to a message signal received from an electroniccomponent such as a sensor. Further, the prior art fails to provide suchtransmitter wherein the piezoelectric layer is electrically connected toa switching element, the switching element for alternately changing theelectrical connections of the transmitter so as to alternately changethe mechanical impedance of the piezoelectric layer. Further, the priorart fails to provide such transducer wherein the mechanical impedance ofthe piezoelectric layer is controlled by providing a plurality ofelectrodes attached thereto, the electrodes being electricallyinterconnected in parallel and anti-parallel electrical connections.Further, the prior art fails to provide such transmitter wherein thepiezoelectric layer features different polarities at distinct portionsthereof. Further, the prior art fails to provide such transmitter whichincludes a chamber containing a low pressure gas for enablingasymmetrical fluctuations of the piezoelectric layer. Further, the priorart fails to provide such transmitter having two-ply piezoelectriclayer.

SUMMARY OF THE INVENTION

[0006] The present invention is of a miniature flexural transducerelement, comprising: (a) a cell element having a cavity; (b) asubstantially flexible piezoelectric layer attached to the cell member,the piezoelectric layer having an external surface and an internalsurface, the piezoelectric layer featuring such dimensions so as toenable fluctuations thereof at its resonance frequency upon impinging ofan external acoustic wave; and (c) a first electrode attached to theexternal surface and a second electrode attached to the internal surfaceof the piezoelectric layer. Preferably, the cavity is etched into asubstrate including an electrically insulating layer and an electricallyconducting layer. The first electrode is preferably integrally made witha substantially thin electrically conducting layer, the electricallyconducting layer being disposed on the substrate and connected theretoby a sealing connection. The cell member may be circular or hexagonal incross section. According to further features in preferred embodiments ofthe invention described below, the substrate may include a plurality ofcell members electrically connected in parallel or serial connections.Preferably, at least one of the electrodes is specifically shaped so asto provide a maximal electrical output, wherein the electrical outputmay be current, voltage or power. A preferred shape of the electrodesincludes two cores interconnected by a connecting member. A transducerelement according to the present invention may also be used as atransmitter.

[0007] Preferably, the cavity of the transducer element includes gas oflow pressure so as to allow its usage as a transmitter. According to thepresent invention there is further provided a transmitter element,comprising: (a) a cell element having a cavity; (b) a substantiallyflexible piezoelectric layer attached to the cell member, thepiezoelectric layer having an external surface and an internal surface,the piezoelectric layer featuring such dimensions so as to enablefluctuations thereof at its resonance frequency upon impinging of anexternal acoustic wave; and (c) a first electrode attached to theexternal surface and a second electrode attached to the internal surfaceof the piezoelectric layer, the electrodes being electrically connectedto an electrical circuit including a switching element for controllablychanging the mechanical impedance of the piezoelectric layer.Preferably, the switching frequency of the switching element equals thefrequency of an electrical message signal arriving from an electronicmember, such as a sensor, thereby modulating a reflected acoustic waveaccording to the frequency of the message signal. The transmitterelement may include a third electrode attached to the external surfaceand a fourth electrode attached to the internal surface of thepiezoelectric layer. When using such a configuration, the switchingelement preferably alternately connects the electrodes in parallel andanti-parallel, thereby controllably changing the mechanical impedance ofthe piezoelectric layer. According to a specific configuration, theelectrodes are interconnected by means of a built-in anti-parallelelectrical connection. Alternatively, the electrodes may beinterconnected by means of a built-in parallel electrical connection.The switching element may be an on/off switch. According to anotherembodiment, the piezoelectric layer includes first and second portionshaving opposite polarities. According to yet another embodiment, thetransmitter element may include two cell members electricallyinterconnected by means of a built-in parallel or anti-parallelelectrical connection. Alternatively, the switching element mayalternately connect the cell members in parallel and anti-parallelelectrical connections. The cell members may have piezoelectric layersof opposite polarities. According to yet another embodiment, the cavityof the transmitter element is covered by a two-ply piezoelectric layerincluding an upper layer bonded to a lower layer. The upper and lowerlayers may feature opposite polarities. The upper and lower layers maybe separated by an insulating layer disposed therebetween. Furtheraccording to the present invention there is provided a method oftransmitting acoustic information, comprising: (a) providing asubstantially flexible piezoelectric layer having first and secondelectrodes attached thereto, the piezoelectric layer being attached to acell member, the electrodes being electrical connected to an electricalcircuit including a switching element; (b) providing an acoustic wavefor impinging on the piezoelectric layer, the acoustic wave having areflected portion; (c) modulating the reflected portion of the acousticwave by controlling the mechanical impedance of the piezoelectric layer,said controlling by switching the switching element at a frequency whichequals the frequency of a message signal arriving from an electroniccomponent such as a sensor. The method may further comprise: (a)providing third and fourth electrodes attached to the piezoelectriclayer, the third and fourth electrodes being electrically connected tothe electrical circuit; (b) changing the electrical connections betweenthe electrodes by means of the switching element so as to change themechanical impedance of the piezoelectric layer. According to a specificconfiguration, the first and second electrodes are attached to a firstcell member and the third and fourth electrodes are attached to a secondcell member.

[0008] The present invention successfully addresses the shortcomings ofthe presently known configurations by providing a miniature flexuralpiezoelectric transducer specifically tailored so as to allow the usageof low frequency acoustic signals for vibrating the piezoelectric layerat its resonant frequency, wherein substantially low frequency signalsherein refer to signals having a wavelength that is much larger thandimensions of the transducer. Further, the present invention addressesthe shortcomings of the presently known configurations by providing suchtransducer element having electrodes specifically shaped so as tomaximize the electrical output of the transducer, and which may beintegrally manufactured with any combination of electronic circuits byusing photolithographic and microelectronics technologies.

[0009] Further, the present invention addresses the shortcomings of thepresently known configurations by providing a miniature flexuralpiezoelectric transmitter which modulates a reflected acoustic wave bycontrollably changing the mechanical impedance of the piezoelectriclayer according to a message signal received from an electroniccomponent such as a sensor. Further, the present invention addresses theshortcomings of the presently known configurations by providing suchtransmitter wherein the mechanical impedance of the piezoelectric layeris controlled by providing a plurality of electrodes attached thereto,the electrodes being interconnected in parallel and anti-parallelelectrical connections, and wherein at least a portion of the electrodesis electrically connected to a switching element, the switching elementfor alternately changing the electrical connections between theelectrodes so as to alternately change the mechanical impedance of thepiezoelectric layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

[0011]FIG. 1 a is a longitudinal section of a transducer elementaccording to the present invention taken along lines A-A in FIGS. 2a-2e;

[0012]FIG. 1b is a longitudinal section of a transducer elementaccording to the present invention taken along lines B-B in FIGS. 2a-2e;

[0013]FIG. 2a is a cross section of a transducer element according tothe present invention taken along line C-C in FIG. 1a;

[0014]FIG. 2b is a cross section of a transducer element according tothe present invention taken along line D-D in FIG. 1 a;

[0015]FIG. 2c is a cross section of a transducer element according tothe present invention taken along line E-E in FIG. 1 a;

[0016]FIG. 2d is a cross section of a transducer element according tothe present invention taken along line F-F in FIG. 1a;

[0017]FIG. 2e is a cross section of a transducer element according tothe present invention taken along line G-G in FIG. 1 a;

[0018]FIG. 3 shows the distribution of charge density across apiezoelectric layer of a transducer element resulting from theapplication of a constant pressure over the entire surface of the layer;

[0019]FIG. 4 shows the results of optimization performed for the powerresponse of a transducer according to the present invention;

[0020]FIG. 5 shows a preferred electrode shape for maximizing the powerresponse of a transducer according to the present invention;

[0021]FIG. 6 is a longitudinal section of another embodiment of atransducer element according to the present invention capable offunctioning as a transmitter;

[0022]FIGS. 7a-7 f are schematic views of possible configurations oftransmitters according to the present invention including parallel andanti-parallel electrical connections for controllably changing themechanical impedance of the piezoelectric layer;

[0023]FIG. 8 is a longitudinal section of a transmitter elementaccording to the present invention including an anti-parallel electricalconnection; and

[0024]FIG. 9 is a longitudinal section of another embodiment of atransmitter element according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention is of a miniature flexural piezoelectrictransducer for receiving acoustic energy transmitted from a remoteacoustic radiation source and converting such energy into electricalpower for activating an electronic circuit.

[0026] Further, the present invention is of a transmitting element andmethod for transmitting information by modulating the reflection of anexternal impinging acoustic wave arrived from a remote transmitter.

[0027] The principles and operation of a transducer element according tothe present invention may be better understood with reference to thedrawings and the accompanying description.

[0028] Referring now to the drawings, FIGS. 1a, 1 b and 2 a-2 eillustrate a preferred embodiment of a transducer element according tothe present invention. As shown in the figures, the transducer element 1includes at least one cell member 3 including a cavity 4 etched into asubstrate and covered by a substantially flexible piezoelectric layer 2.Attached to piezoelectric layer 2 are an upper electrode 8 and a lowerelectrode 6, the electrodes for connection to an electronic circuit.

[0029] The substrate is preferably made of an electrical conductinglayer 11 disposed on an electrically insulating layer 12, such thatcavity 4 is etched substantially through the thickness of electricallyconducting layer 11.

[0030] Electrically conducting layer 11 is preferably made of copper andinsulating layer 12 is preferably made of a polymer such as polyimide.Conventional copper-plated polymer laminate such as Kapton™ sheets maybe used for the production of transducer element 1. Commerciallyavailable laminates such as Novaclad™ may be used. Alternatively, thesubstrate may include a silicon layer, or any other suitable material.Alternatively, layer 11 is made of a non-conductive material such asPyralin™.

[0031] Preferably, cavity 4 is etched into the substrate by usingconventional printed-circuit photolithography methods. Alternatively,cavity 4 may be etched into the substrate by using VLSI/micro-machiningtechnology or any other suitable technology.

[0032] Piezoelectric layer 2 may be made of PVDF or a copolymer thereof.Alternatively, piezoelectric layer 2 is made of a substantially flexiblepiezoceramic. Preferably, piezoelectric layer 2 is a poled PVDF sheethaving a thickness of about 9-28 μm.

[0033] Preferably, the thickness and radius of flexible layer 2, as wellas the pressure within cavity 4, are specifically selected so as toprovide a predetermined resonant frequency. When using the embodiment ofFIGS. 1a and 1 b, the radius of layer 2 is defined by the radius ofcavity 4.

[0034] By using a substantially flexible piezoelectric layer 2, thepresent invention allows to provide a miniature transducer element whoseresonant frequency is such that the acoustic wavelength is much largerthan the extent of the transducer. This enables the transducer to beomnidirectional even at resonance, and further allows the use ofrelatively low frequency acoustic signals which do not suffer fromsignificant attenuation in the surrounding medium.

[0035] Prior art designs of miniature transducers, however, rely onrigid piezoceramic usually operating in thickness mode. In such casesthe resonant frequency relates to the size of the element and speed ofsound in the piezoceramic, and is higher by several orders of magnitude.

[0036] The present invention provides a transducer which isomnidirectional, i.e., insensitive to the direction of the impingingacoustic rays, thereby substantially simplifying the transducer'soperation relative to other resonant devices. Such a transducer elementis thus suitable for application in confined or hidden locations, wherethe orientation of the transducer element cannot be ascertained inadvance.

[0037] According to a specific embodiment, cavity 4 features a circularor hexagonal shape with radius of about 200 μm. Electrically conductinglayer 11 preferably has a thickness of about 15 μm. Cell member 3 ispreferably etched completely through the thickness of electricallyconducting layer 11. Electrically insulating layer 12 preferablyfeatures a thickness of about 50 μm. The precise dimensions of thevarious elements of a transducer element according to the presentinvention may be specifically tailored according to the requirements ofthe specific application.

[0038] Cavity 4 preferably includes a gas such as air. The pressure ofgas within cavity 4 may be specifically selected so as to predeterminethe sensitivity and ruggedness of the transducer as well as the resonantfrequency of layer 2.

[0039] As shown in FIG. 2b, an insulating chamber 18 is etched into thesubstrate, preferably through the thickness of conducting layer 11, soas to insulate the transducer element from other portions of thesubstrate which may include other electrical components such as othertransducer elements etched into the substrate. According to a specificembodiment, the width of insulating chamber 18 is about 100 um. Asshown, insulating chamber 18 is etched into the substrate so as to forma wall 10 of a predetermined thickness enclosing cavity 4, and aconducting line 17 integrally made with wall 10 for connecting thetransducer element to another electronic component preferably etchedinto the same substrate, or to an external electronic circuit.

[0040] As shown in FIGS. 1a and 1 b, attached to piezoelectric layer 2are upper electrode 8 and lower electrode 6. As shown in FIGS. 2c and 2e, upper electrode 8 and lower electrode 6 are preferably preciselyshaped so as to cover a predetermined area of piezoelectric layer 2.Electrodes 6 and 8 may be deposited on the upper and lower surfaces ofpiezoelectric membrane 2, respectively, by using various methods such asvacuum deposition, mask etching, painting, and the like.

[0041] As shown in FIG. 1a, lower electrode 6 is preferably made as anintegral part of a substantially thin electrically conducting layer 14disposed on electrically conducting layer 11. Preferably, electricallyconducting layer 14 is made of a Nickel-Copper alloy and is attached toelectrically conducting layer 11 by means of a sealing connection 16.Sealing connection 16 may be made of indium. According to a preferredconfiguration, sealing connection 16 may feature a thickness of about 10m, such that the overall height of wall 10 of cavity 4 is about 20-25μm.

[0042] As shown in FIG. 2c, electrically conducting layer 14 covers thevarious portions of conducting layer 11, including wall 10 andconducting line 17. The portion of conducting layer 14 coveringconducting line 17 is for connection to an electronic component such asa neighboring cell.

[0043] According to a preferred embodiment of the present invention,electrodes 6 and 8 are specifically shaped to include the mostenergy-productive region of piezoelectric layer 2 so as to providemaximal response of the transducer while optimizing the electrode area,and therefore the cell capacitance, thereby maximizing a selectedparameter such as voltage sensitivity, current sensitivity, or powersensitivity of the transducer element.

[0044] The vertical displacement of piezoelectric layer 2, Ψ, resultingfrom a monochromatic excitation at angular frequency ω is modeled usingthe standard equation for thin plates:${{\left( {\nabla^{2}{- \gamma^{2}}} \right)\left( {\nabla^{2}{+ \gamma^{2}}} \right)\Psi} - {\frac{3\left( {1 - v^{2}} \right)}{2Q\quad h^{3}}P} + {\frac{3i\quad Z\quad {\omega \left( {1 - v^{2}} \right)}}{2Q\quad h^{3}}\overset{\_}{\Psi}}} = 0$

[0045] wherein Q is the Young's modulus representing the elasticity oflayer 2; h the half-thickness of layer 2; ν is the Poisson ratio forlayer 2; γ is the effective wavenumber in the layer given by:γ⁴=3ρ(1−ν²)ω²/Qh², wherein ρ is the density of layer 2 and ω is theangular frequency of the applied pressure (wherein the applied pressuremay include the acoustic pressure, the static pressure differentialacross layer 2 and any other pressure the transducer comes across); Z isthe mechanical impedance resulting from the coupling of layer 2 to bothexternal and internal media of cavity 4, wherein the internal medium ispreferably air and the external medium is preferably fluid; P is theacoustic pressure applied to layer 2, and {overscore (Ψ)} represents theaverage vertical displacement of layer 2.

[0046] When chamber 4 is circular, the solution (given for a singlefrequency component ω) representing the dynamic displacement of acircular layer 2 having a predetermined radius a, expressed in polarcoordinates, is:${\Psi \left( {r,\phi} \right)} = {\frac{{{I_{1}\left( {\gamma \quad a} \right)}\left\lbrack {{J_{0}\left( {\gamma \quad r} \right)} - {J_{0}\left( {\gamma \quad a} \right)}} \right\rbrack} + {{J_{1}\left( {\gamma \quad a} \right)}\left\lbrack {{I_{0}\left( {\gamma \quad r} \right)} - {I_{0}\left( {\gamma \quad a} \right)}} \right\rbrack}}{{2h\quad {\rho\omega}^{2}{L_{0}\left( {\gamma \quad a} \right)}} + {{\omega}\quad Z\quad {L_{2}\left( {\gamma \quad a} \right)}}}P}$L₀(z) = I₀(z)J₁(z) + J₀(z)I₁(z), L₂(z) = J₂(z)I₁(z) − I₂(z)J₁(z)$Z = {\frac{P_{A}}{{\omega}\quad H_{A}} + {{i\left\lbrack {\frac{4}{3\pi} + \frac{1}{6}} \right\rbrack}{\omega\rho}_{W}a}}$

[0047] wherein:

[0048] Ψ(r,φ) is time-dependent and represents the displacement of aselected point located on circular layer 2, the specific location ofwhich is given by radius r and angle φ;

[0049] J and I are the normal and modified Bessel functions of the firstkind, respectively; P_(A), H_(A) are the air pressure within cavity 4and the height of chamber 4, respectively; and ρ_(w) is the density ofthe fluid external to cavity 4.

[0050] The first term of the impedance Z relates to the stiffnessresulting from compression of air within cavity 4, and the second termof Z relates to the mass added by the fluid boundary layer. Anadditional term of the impedance Z relating to the radiated acousticenergy is substantially negligible in this example.

[0051] The charge collected between electrodes 6 and 8 per unit area isobtained by evaluating the strains in layer 2 resulting from thedisplacements, and multiplying by the pertinent off-diagonal elements ofthe piezoelectric strain coefficient tensor, e₃₁, e₃₂, as follows:${Q\left( {r,\phi,t} \right)} = {{e_{31}\left( \frac{\partial\Psi}{\partial x} \right)}^{2} + {e_{32}\left( \frac{\partial\Psi}{\partial y} \right)}^{2}}$

[0052] wherein:

[0053] Q(r,φ,t) represents the charge density at a selected pointlocated on circular layer 2, the specific location of which is given byradius r and angle φ;

[0054] x is the stretch direction of piezoelectric layer 2; y is thetransverse direction (the direction perpendicular to the stretchdirection) of layer 2;

[0055] e₃₁, e₃₂ are off-diagonal elements of the piezoelectric straincoefficient tensor representing the charge accumulated at a selectedpoint on layer 2 due to a given strain along the x and y directions,respectively, which coefficients being substantially dissimilar whenusing a PVDF layer.

[0056] Ψ is the displacement of layer 2, taken as the sum of thedisplacement for a given acoustic pressure P at frequency f, and thestatic displacement resulting from the pressure differential between theinterior and exterior of cavity 4, which displacements being extractablefrom the equations given above.

[0057] The total charge accumulated between electrodes 6 and 8 isobtained by integrating Q(r,φ, t) over the entire area S of theelectrode:$Q = {\int_{S}{{Q\left( {r,\phi,t} \right)}{\overset{\rho}{x}}}}$

[0058] The capacitance C of piezoelectric layer 2 is given by:${C = {\frac{ɛ}{2h}{\int_{S}{\overset{\rho}{x}}}}},$

[0059] wherein ε is the dielectric constant of piezoelectric layer 2;and 2 h is the thickness of piezoelectric layer 2.

[0060] Accordingly, the voltage, current and power responses ofpiezoelectric layer 2 are evaluated as follows:${V = \frac{2h{\int_{S}{{Q\left( {r,\phi,t} \right)}{\overset{\rho}{x}}}}}{ɛ{\int_{S}{\overset{\rho}{x}}}}},{I = {2{\omega}{\int_{S}{{Q\left( {r,\phi,t} \right)}{\overset{\rho}{x}}}}}},{W = \frac{4i\quad {h\left\lbrack {{\int_{S}{{Q\left( {r,\phi,t} \right)}{\overset{\rho}{x}}}},} \right\rbrack}^{2}}{ɛ{\int_{S}{\overset{\rho}{x}}}}}$

[0061] The DC components of Q are usually removed prior to theevaluation, since the DC currents are usually filtered out. The valuesof Q given above represent peak values of the AC components of Q, andshould be modified accordingly so as to obtain other required valuessuch as RMS values.

[0062] According to the above, the electrical output of the transducerexpressed in terms of voltage, current and power responses depend on theAC components of Q, and on the shape S of the electrodes. Further, ascan be seen from the above equations, the voltage response of thetransducer may be substantially maximized by minimizing the area of theelectrode. The current response, however, may be substantially maximizedby maximizing the area of the electrode.

[0063]FIG. 3 shows the distribution of charge density on a circularpiezoelectric layer 2 obtained as a result of pressure (acoustic andhydrostatic) applied uniformly over the entire area of layer 2, whereinspecific locations on layer 2 are herein defined by using Cartesiancoordinates including the stretch direction (x direction) and thetransverse direction (y direction) of layer 2. It can be seen thatdistinct locations on layer 2 contribute differently to the chargedensity. The charge density vanishes at the external periphery 70 and atthe center 72 of layer 2 due to minimal deformation of these portions.The charge density is maximal at two cores 74 a and 74 b locatedsymmetrically on each side of center 72 due to maximal strains (in thestretch direction) of these portions.

[0064] A preferred strategy for optimizing the electrical responses ofthe transducer is to shape the electrode by selecting the areascontributing at least a selected threshold percentage of the maximalcharge density, wherein the threshold value is the parameter to beoptimized. A threshold value of 0% relates to an electrode covering theentire area of layer 2.

[0065]FIG. 4 shows the results of an optimization performed for thepower response of a transducer having a layer 2 of a predetermined area.As shown in the figure, the threshold value which provides an optimalpower response is about 30% (graph b). Accordingly, an electrode whichcovers only the portions of layer 2 contributing at least 30% of themaximal charge density yields a maximal power response. The pertinentvoltage response obtained by such an electrode is higher by a factor of2 relative to an electrode completely covering layer 2 (graph a). Thecurrent response obtained by such electrode is slightly lower relativeto an electrode completely covering layer 2 (graph c). Further as shownin the figure, the deflection of layer 2 is maximal when applying anacoustic signal at the resonant frequency of layer 2 (graph d).

[0066] A preferred electrode shape for maximizing the power response ofthe transducer is shown in FIG. 5, wherein the electrode includes twoelectrode portions 80 a and 80 b substantially covering the maximalcharge density portions of layer 2, the electrode portions beinginterconnected by means of a connecting member 82 having a minimal area.Preferably, portions 80 a and 80 b cover the portions of layer 2 whichyield at least a selected threshold (e.g. 30%) of the maximal chargedensity.

[0067] According to the present invention any other parameter may beoptimized so as to determine the shape of electrodes 6 and 8. Accordingto further features of the present invention, only one electrode (upperelectrode 8 or lower electrode 6) may be shaped so as to provide maximalelectrical response of the transducer, with the other electrode coveringthe entire area of layer 2. Since the charge is collected only at theportions of layer 2 received between upper electrode 8 and lowerelectrode 6, such configuration is operatively equivalent to aconfiguration including two shaped electrodes having identical shapes.

[0068] Referring now to FIG. 6, according to another embodiment of thepresent invention chamber 4 of transducer element 1 may contain gas ofsubstantially low pressure, thereby conferring a substantially concaveshape to piezoelectric membrane 2 at equilibrium. Such configurationenables to further increase the electrical response of the transducer byincreasing the total charge obtained for a given displacement of layer2. The total displacement in such an embodiment is given by:Ψ=P₀Ψ_(DC)+PΨ_(AC) cosωt, wherein P₀ is the static pressure differentialbetween the exterior and the interior of cavity 4; Ψ_(DC) is thedisplacement resulting from P₀; P is the amplitude of the acousticpressure; and Ψ_(DC) the displacement resulting from P.

[0069] Accordingly, the strain along the x direction includes threeterms as follows:$S_{x\quad x} = {\left( \frac{\partial\Psi}{\partial x} \right)^{2} = {{P_{0}^{2}\left( \frac{\partial\Psi_{D\quad C}}{\partial x} \right)}^{2} + {{P^{2}\left( \frac{\partial\Psi_{A\quad C}}{\partial x} \right)}^{2}\cos^{2}\omega \quad t} + {2P_{0}P\frac{\partial\Psi_{D\quad C}}{\partial x}\frac{\partial\Psi_{A\quad C}}{\partial x}\cos \quad \omega \quad t}}}$

[0070] wherein the DC component is usually filtered out.

[0071] Thus, by decreasing the pressure of the medium (preferably air)within cavity 4 relative to the pressure of the external medium(preferably fluid), the value of P₀ is increased, thereby increasing thevalue of the third term of the above equation.

[0072] Such embodiment of the present invention makes it possible toincrease the charge output of layer 2 for a given displacement, therebyincreasing the voltage, current and power responses of the transducerwithout having to increase the acoustic pressure P. Further, suchembodiment enables to further miniaturize the transducer since the sameelectrical response may obtain for smaller acoustic deflections. Suchembodiment is substantially more robust mechanically and therefore moredurable than the embodiment shown in FIGS. 1a and 1 b. Such furtherminiaturization of the transducer enables to use higher resonancefrequencies relative to the embodiment shown in FIGS. 1a and 1 b.

[0073] Preferably, a transducer element 1 according to the presentinvention is fabricated by using technologies which are in wide use inthe microelectronics industry so as to allow integration thereof withother conventional electronic components. When the transducer elementincludes a substrate such as Copper-polymer laminate or silicon, avariety of conventional electronic components may be fabricated onto thesame substrate.

[0074] According to the present invention, a plurality of cavities 4 maybe etched into a single substrate 12 and covered by a singlepiezoelectric layer 2 so as to provide a transducer element including amatrix of transducing cells members 3, thereby providing a larger energycollecting area of predetermined dimensions while still retaining theadvantage of miniature individual transducing cell members 3. When usingsuch configuration, the transducing cell members 3 may be electricallyinterconnected in parallel or serial connections, or combinationsthereof, so as to tailor the voltage and current response of thetransducer. Parallel connections are preferably used so as to increasethe current output while serial connections are preferably used so as toincrease the voltage output of the transducer.

[0075] Further, piezoelectric layer 2 may be completely depolarized andthen repolarized at specific regions thereof so as to provide apredetermined polarity to each of the transducing cell members 3. Suchconfiguration enables to reduce the complexity of interconnectionsbetween the cell members 3.

[0076] A transducer element according to the present invention may befurther used as a transmitter for transmitting information to a remotereceiver by modulating the reflection of an external impinging acousticwave arrived from a remote transmitter.

[0077] Referring to FIG. 6, the transducer element shown may function asa transmitter element due to the asymmetric fluctuations ofpiezoelectric layer 2 with respect to positive and negative transientacoustic pressures obtained as a result of the pressure differentialbetween the interior and exterior of cavity 4.

[0078] A transmitter element according to the present inventionpreferably modulates the reflection of an external impinging acousticwave by means of a switching element connected thereto. The switchingelement encodes the information that is to be transmitted, such as theoutput of a sensor, thereby frequency modulating a reflected acousticwave.

[0079] Such configuration requires very little expenditure of energyfrom the transmitting module itself, since the acoustic wave that isreceived is externally generated, such that the only energy required fortransmission is the energy of modulation.

[0080] Specifically, the reflected acoustic signal is modulated byswitching the switching element according to the frequency of a messageelectric signal arriving from another electronic component such as asensor, so as to controllably change the mechanical impedance of layer 2according to the frequency of the message signal.

[0081] Preferably, the invention uses a specific array of electrodesconnected to a single cell member 3 or alternatively to a plurality ofcell members so as to control the mechanical impedance of layer 2.

[0082]FIGS. 7a-7 g illustrate possible configurations for controllablychange the impedance of layer 2 of a transmitter element. Referring toFIG. 7a, a transmitter element according to the present invention mayinclude a first and second pairs of electrodes, the first pair includingan upper electrode 40 a and a lower electrode 38 a, and the second pairincluding an upper electrode 40 b and a lower electrode 38 b. Electrodes38 a, 38 b, 40 a and 40 b are electrically connected to an electricalcircuit by means of conducting lines 36 a, 36 b, 34 a and 34 b,respectively, the electrical circuit including a switching element (notshown) so as to alternately change the electrical connections ofconducting lines 36 a, 36 b, 34 a and 34 b.

[0083] Preferably, the switching element switches between a parallelconnection and an anti-parallel connection of the electrodes. A parallelconnection decreases the mechanical impedance of layer 2, wherein ananti-parallel connection increases the mechanical impedance of layer 2.An anti-parallel connection may be obtained by interconnecting line 34 ato 36 b and line 34 b to 36 a. A parallel connection may be obtained byconnecting line 34 a to 34 b and line 36 a to 36 b. Preferably, theswitching frequency equals the frequency of a message signal arrivingfrom an electrical component such as a sensor.

[0084] According to another embodiment (FIG. 7b), upper electrode 40 ais connected to lower electrode 38 b by means of a conducting line 28,and electrodes 38 a and 40 b are connected to an electrical circuit bymeans of conducting lines 27 and 29, respectively, the electricalcircuit including a switching element. Such configuration provides ananti-parallel connection of the electrodes, wherein the switchingelement functions as an on/off switch, thereby alternately increasingthe mechanical impedance of layer 2.

[0085] In order to reduce the complexity of the electrical connections,layer 2 may be depolarized and then repolarized at specific regionsthereof. As shown in FIG. 7c, the polarity of the portion of layer 2received between electrodes 40 a and 38 a is opposite to the polarity ofthe portion of layer 2 received between electrodes 40 b and 38 b. Ananti-parallel connection is thus achieved by interconnecting electrodes38 a and 38 b by means of a conducting line 28, and providing conductinglines 27 and 29 connected to electrodes 40 a and 40 b, respectively, theconducting lines for connection to an electrical circuit including aswitching element.

[0086] According to another embodiment, the transmitting elementincludes a plurality of transducing cell members, such that themechanical impedance of layer 2 controllably changed by appropriatelyinterconnecting the cell members.

[0087] As shown in FIG. 7d, a first transducing cell member 3 aincluding a layer 2 a and a cavity 4 a, and a second transducing cellmember 3 b including a layer 2 b and a cavity 4 b are preferablycontained within the same substrate; and layers 2 a and 2 b arepreferably integrally made (not shown). A first pair of electrodesincluding electrodes 6 a and 8 a is attached to layer 2, and a secondpair of electrode including electrodes 6 b and 8 b is attached to layer2 b. Electrodes 6 a, 8 a, 6 b and 8 b are electrically connected to anelectrical circuit by means of conducting lines 37 a, 35 a, 37 b and 35b, respectively, the electrical circuit including a switching element soas to alternately switch the electrical connections of conducting lines37 a, 35 a, 37 b and 35 b so as to alternately provide parallel andanti-parallel connections, substantially as described for FIG. 7a,thereby alternately decreasing and increasing the mechanical impedanceof layers 2 a and 2 b.

[0088]FIG. 7e illustrates another embodiment, wherein the first andsecond transducing cell members are interconnected by means of ananti-parallel connection. As shown in the figure, the polarity of layer2 a is opposite to the polarity of layer 2 b so as to reduce thecomplexity of the electrical connections between cell members 3 a and 3b. Thus, electrode 6 a is connected to electrode 6 b by means of aconducting line 21, and electrodes 8 a and 8 b are provided withconducting lines 20 and 22, respectively, for connection to anelectrical circuit including a switching element, wherein the switchingelement preferably functions as an on/off switch so as to alternatelyincrease the mechanical impedance of layers 2 a and 2 b.

[0089]FIG. 7f shows another embodiment, wherein the first and secondtransducing cell members are interconnected by means of a parallelconnection. As shown, electrodes 6 a and 6 b are interconnected by meansof conducting line 24, electrodes 8 a and 8 b are interconnected bymeans of conducting line 23, and electrodes 6 b and 8 b are providedwith conducting lines 26 and 25, respectively, the conducting lines forconnection to an electrical circuit including a switching element. Theswitching element preferably functions as an on/off switch foralternately decreasing and increasing the mechanical impedance of layers2 a and 2 b.

[0090]FIG. 8 shows a possible configuration of two transducing cellmembers etched onto the same substrate and interconnected by means of ananti-parallel connection. As shown in the figure, the transducing cellmembers are covered by a common piezoelectric layer 2, wherein thepolarity of the portion of layer 2 received between electrodes 6 a and 8a is opposite to the polarity of the portion of layer 2 received betweenelectrodes 6 b and 8 b. Electrodes 8 a and 8 b are bonded by means of aconducting line 9, and electrodes 6 a and 6 b are provided withconducting lines 16 for connection to an electrical circuit.

[0091] Another embodiment of a transmitter element according to thepresent invention is shown in FIG. 9. The transmitter element includes atransducing cell member having a cavity 4 covered by a first and secondpiezoelectric layers, 50 a and 50 b, preferably having oppositepolarities. Preferably, layers 50 a and 50 b are interconnected by meansof an insulating layer 52. Attached to layer 50 a are upper and lowerelectrodes 44 a and 42 a, and attached to layer 50 b are upper and lowerelectrodes 44 b and 42 b. Electrodes 44 a, 42 a, 44 b and 42 b areprovided with conducting lines 54, 55, 56 and 57, respectively, forconnection to an electrical circuit.

[0092] It will be appreciated that the above descriptions are intendedonly to serve as examples, and that many other embodiments are possiblewithin the spirit and the scope of the present invention.

What is claimed is:
 1. A transducer element, comprising: (a) a cellmember having a cavity; (b) a substantially flexible piezoelectric layerattached to said cell member, said piezoelectric layer having anexternal surface and an internal surface, said piezoelectric layerfeaturing such dimensions so as to enable fluctuations thereof at itsresonance frequency upon impinging of an external acoustic wave; and (c)a first electrode attached to said external surface and a secondelectrode attached to said internal surface.
 2. The transducer elementof claim 1, wherein the wavelength of said acoustic wave issubstantially larger than said dimensions.
 3. The transducer element ofclaim 2, wherein said cavity is etched into a substrate.
 4. Thetransducer element of claim 3, where said substrate includes anelectrically insulating layer and an electrically conducting layer. 5.The transducer element of claim 4, wherein said first electrode isintegrally made with a substantially thin electrically conducting layerdisposed on said substrate.
 6. The transducer element of claim 5,wherein said substantially thin electrically conducting layer isconnected to said substrate by means of a sealing connection.
 7. Thetransducer element of claim 3, wherein said substrate includes aplurality of cell members.
 8. The transducer element of claim 7, whereinsaid plurality of cell members are electrically connected in parallelconnections.
 9. The transducer element of claim 7, wherein saidplurality of cell members are electrically connected in serialconnections.
 10. The transducer element of claim 1, wherein said cellmember is circular in shape.
 11. The transducer element of claim 2,wherein said cavity includes a gas.
 12. The transducer element of claim11, wherein said gas is of substantially low pressure.
 13. Thetransducer element of claim 2, wherein said transducer element is usedas a transmitter.
 14. The transducer element of claim 13, furtherincluding a switching element electrically connected thereto so as tocontrollably change the mechanical impedance of said piezoelectriclayer.
 15. A transmitter element, comprising: (a) a cell member having acavity; (b) a substantially flexible piezoelectric layer attached tosaid cell member, said piezoelectric layer having an external surfaceand an internal surface, said piezoelectric layer featuring suchdimensions so as to enable fluctuations thereof at its resonancefrequency upon impinging of an external acoustic field; and (c) a firstelectrode attached to said external surface and a second electrodeattached to said internal surface, said electrodes coupled to anelectrical circuit, said electrical circuit having a switching element,whereby a switching frequency of said switching element controls themechanical impedance of the piezoelectric layer.
 16. The transmitterelement of claim 15, wherein said cavity is isolated.
 17. Thetransmitter element of claim 15, wherein the switching frequency of saidswitching element equals the frequency of an electrical message signalarriving from an electric member.
 18. The transmitter element of claim17, wherein said electronic member is a sensor.
 19. The transmitterelement of claim 15, wherein said switching element is for modulating areflected acoustic wave according to a message signal arriving from anelectronic component.
 20. The transmitter element of claim 15, furtherincluding a third electrode attached so said external surface and afourth electrode attached to said internal surface.
 21. The transmitterelement of claim 20, wherein said switching element alternately connectssaid electrodes in parallel and anti-parallel connections, therebycontrollably changing the mechanical impedance of said piezoelectriclayer.
 22. The transmitter element of claim 20, wherein said electrodesare electrically interconnected by means of a substantially built-inanti-parallel connection.
 23. The transmitter element of claim 20,wherein said electrodes are electrically interconnected by means of asubstantially built-in parallel connection.
 24. The transmitter elementof claim 20, wherein said switching element is an on/off switch.
 25. Thetransmitter element of claim 20, wherein said piezoelectric layerincludes first and second portions having opposite polarities.